1. Field of the Invention
This invention relates generally to bolometer type radiation sensors for detecting thermal radiation and more particularly to circuitry for providing a readout signal of a bolometer sensor.
2. Related Art
Bolometers are well known in the art and comprise devices which generate a voltage output when thermal radiation is absorbed. These devices have been successfully used for infra-red (IR) imaging in the long wave infra-red (LWIR) band of the electromagnetic spectrum. Extending these devices to other spectral bands has proven relatively difficult in the past. However, efforts are currently under way to extend this capability to millimeter wave (mm) and terahertz (THz) spectral bands and thus there is a need for imagers operating in the mm and THz spectral bands. Applications for such devices include, for example, multi-spectral imaging for improved navigation, target recognition and detection as well as homeland defense applications. Such applications all require the use of bolometers. Therefore, realizing bolometers with acceptable performance with mm-THZ-LWIR cameras requires the formulation of new approaches for overcoming conventional limitations such as the requirement for faster response time and the ability to maintain sensitivity for relatively long periods. Moreover, fast response time dictates minimizing the mass of the bolometer's absorbing element.
In related application Ser. No. 11/239,297 (Northrop Grumman Ref. No. 000800-078), entitled “Sensitive Silicon Sensor And Test Structure For An Ultra-Sensitive Silicon Sensor”, there is disclosed a sensor of thermal radiation comprised of a pair of silicon diodes connected in back-to-back relationship with one of the diodes being located in a detector stage, while the other diode is located in a heat bath stage along with a temperature difference amplifier. The detector stage is thermally isolated from the heat bath stage by a low thermal conductivity link which includes electrical wiring for connecting the back-to-back diodes.
In related application Ser. No. 11/239,275 (Northrop Grumman Ref. No. 000775-078), entitled “Focal Plane Antenna To Sensor Interface For An Ultra-Sensitive Silicon Sensor”, there is disclosed an electrical interface between a scene to be imaged, and a bolometer type sensor located, for example in a pixel, and wherein the efficiency of each pixel is improved by means of a thermal energy concentrator including a lens and an antenna. Where a plurality of pixels are located in an array, a microantenna is provided for each pixel in the array with a common lens being provided to focus and channel incoming radiation to each microantenna. Radiation from a scene is further coupled by means of a lens and microantenna to the absorbing element of each bolometer through an AC coupling circuit including an electronic chopper implemented by means of a PIN diode, the conductivity of which is varied so as to affect the reflection coefficient of the input signal supplied through the microantenna.
In U.S. Pat. No. 6,489,615, there is disclosed in a pair of back-to-back temperature sensing silicon diodes respectively located in a detector stage and an intermediate stage and coupled to a temperature difference amplifier also located in the intermediate stage. The intermediate stage is located between the detector stage and the heat bath stage, with the intermediate stage also including an electro-thermal feedback loop which is provided by the heat generated by an amplifier located in the intermediate stage which generates heat which is proportional to the temperature difference between the difference between the detected temperatures provided by the silicon diodes. The heat provided by the amplifier acts to actively zero the temperature difference between the detector and the intermediate stage so as to eliminate any net heat flow between the detector element and the intermediate stage.